cleanup: move declarations of external objects into header files

It is not a good idea to declare external objects in .c files.  Every
external object (ie, object with external linkage) should be declared
exactly once in a .h file, and every .c file that refers to it or
defines it should #include that header.

When combined with appropriate compiler warnings, this ensures that
every file sees the same signature for every such object.  (At least
for functions.)

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>